Mike,
They're all right.
For easier starts in cold weather use the .016. For everything else, use .018. Going to the bigger gap is OK if your plugs are clean and you are getting a good strong spark.
With plug gaps, smaller is safer. Too much gap with a dirty plug or weak mag and you get no spark.
If you set a plug at .018 and run it a long time, you might even burn it out to .020.
I'll throw one more monkey wrench in the works. If I set a plug at .018 and you set a plug at .018, I'd just about guarantee that they wont be the same, but they'll both be within the max. and min. limits.
Use the middle figure and you'll be fine.
Bill.
